Friday, October 17, 2008

Understanding Moon Phases

Take the time to observe the moon through all of its phases. When the moon is about three or four days “old,” Mare Crisium and other vivid features—including the prominent craters Burckhardt and Geminus—become dramatically visible, assuming it’s a clear night. You can also begin to see Mare Tranquilitatis, the Sea of Tranquility.At day seven, the moon is at its first quarter. At this time, mountains and craters are most dramatically visible. Indeed, this is the optimum night for looking at lunar features in their most deeply shadowed relief.As the moon enters its waxing gibbous beyond first quarter phase, its full, bright light is cheerful, but so bright that it actually becomes more difficult to make out sharp details on the lunar surface. An inexpensive “moon filter” or variable polarizing filter fitted to your telescope can help increase contrast on the bright lunar surface. As the moon verges on full, we do get great views of the eastern maria, the lunar plains. Past day 14, the moon begins to wane as the sunset terminator moves slowly across the lunar landscape. At about day 22, the Apennine Mountains are clearly visible. It was these mountains that Galileo studied most intensely, attempting to judge their height by the shadows they cast.During the late waning phase of the moon, moonrise comes later and later at night, as the moon gradually catches up with the sun in the sky. By the time the moon passes day 26, it is nothing but a thin crescent of light present in the predawn sky. The new moon follows, and as the moon overtakes the sun, the crescent reappears (on the other side of the moon at sunset), and it begins to wax again. Here are some cold, hard facts about a cold, hard place. The moon is Earth’s only natural satellite, and in fact a very large satellite for a planet as small as the earth. The planet Mercury is only slightly larger than the moon. The mean distance between the earth and moon, as it orbits our planet from west to east, is 239,900 miles (386,239 km). The moon is less than one-third the size of the earth, with a diameter of about 2,160 miles (3,476 km) at its equator. Moreover, it is much less massive and less dense than the earth—1⁄80 as massive, with a density of 3.34 g/cm3, in contrast to 5.52 g/cm3 for the earth. If the earth were the size of your head, the orbiting moon would be a tennis ball 30 feet away.Because the moon is so much less massive than the earth, and about a third as big, its surface gravity is about one-sixth that of our planet. That’s why the Apollo astronauts could skip and jump like they did, even wearing those heavy space suits. If you weigh 160 pounds on the earth’s surface, you would weigh only 27 pounds on the moon. This apparent change would give you the feeling of having great strength, since your body’s muscles are accustomed to lifting and carrying six times the load that burdens them on the moon. Of course, your mass—how much matter is in you—does not change. If your mass is 60 kilograms (kg) on the earth, it will still be 60 kg on the moon. the moon is in a synchronous orbit around the earth; that is, it rotates once on its axis every 27.3 days, which is the same time that it takes to complete one orbit around the earth. Thus synchronized, we see only one side of the moon (except for the tantalizing peek at the far side that libration affords).